(2x + 29) + 91 = 180.
This is because they add to form a straight line which is 180 degrees, in other words the angles are supplementary.
2x + 29 = 89 (Subtract 91 from each side)
2x = 60 (Subtract 29 from each side)
x = 30 (Divide 2 from each side)
x = 30
The answer is B